2 Answers
2

Plue of Rave Master is a stout, white creature, reminiscent of a snowman, that only comes up to three feet in height. Plue was born in the Kingdom of Symphonia, signifying he is a creature of some sort. (Source: Rave Master Wiki)

In contrast, Plue aka Nikora, "The Canis Minor" (ニコラ Nikora) is a Silver Key Celestial Spirit that has no obvious combat abilities. It is stated to usually be kept as a pet for low-powered Mages who is in contract with Lucy Heartfilia. (Source: Fairy Tail Wiki)

Since both series have the same author, Mashima must've decided to use his own design again. But they are different, as one of them is a creature, while the other is a celestial spirit.